The essential atmospheric agents for microwave radiometry are oxygen and water vapor. The atmospheric oxygen and water vapor emit on a cloudless sky thermal noise and provide the so called clear sky radiometric brightness temperature T.sub.sky. Inspected from the ground, the atmospheric brightness temperature at clear weather is a function of both frequency and elevation. The frequency dependence is due to the resonant absorption/emission spectrum of water and oxygen molecules. Due to the atmospheric pressure the spectral lines are spread on a broader frequency range. The lowest spectral line of the water molecule absorption/emission resonance is at approx. 22 GHz frequency (FIG. 6a). The elevation angle dependence of the clear sky brightness temperature results from geometry. The transmission path length of the layer formed on the ground by the atmosphere is considerably shorter in the zenith direction than closer to the horizon (FIG. 6b). The radiometric brightness temperature of the atmosphere is to a certain extent dependent on the amount of effective agent in the radiometer beam, at clear sky on the so called effective path length of the inspection direction. The clear sky brightness temperature in zenith direction is thus considerably lower than close to the horizon (FIG. 6a).
Water is present in the atmosphere in water vapor and liquid form and as ice in clouds and rains. Atmospheric water content changes: air humidity, clouds and rain occur in the microwave region as changes in the sky brightness temperature.
Atmospheric property observations with a radiometric scanner and rain indication with a rain detector are presented as examples of the fields of embodiment of the method and device according to the invention.
Atmospheric and ground properties have been measured by microwave radiometers both from satellites (weather and remote sensing satellites) and from the ground.
Atmospheric microwave radiometric measurements from the ground have been utilized for example, in meteorological applications, in measurements relating to interferometric and electromagnetic wave propagation studies, e.g.:
i) the U.S. Pat. No. 4,873,481 PA1 ii) Measurement of atmospheric water vapor with microwave radiometry; S. Elgered et al./Chalmers University of Technology, Sweden, PA1 iii) Utilization of the radiometry method in a satellite connection propagation study; T. Kokkila, thesis for diploma, University of Technology 1988, PA1 iv) Correction of satellite beacon propagation data using radiometer measurements; Stutzman, Haidara, Reklus, IEEE Proceedings.-Microwaves. Antennas. Propagation, Vol 141 No.1 Feb 1994, PA1 v) Use of radiometers in atmospheric attenuation measurements Allnut, Pratt, Stutzman, Snider IEEE Proceedings.- Microwaves. Antennas. Propagation, Vol 141 No.5 Oct 1994.
The measuring device used in the above references are radiometers of Dicke-type (i, ii, iii) and a total output radiometer (iv, v). The radiometers are multichannel or connected to a measuring system utilizing radiometric measurements was to determine atmospheric properties by brightness temperature absolute values. The measuring of the brightness temperature absolute values requires stabilization of the radiometer amplification, measuring result calibration, accurate knowledge of the antenna side lobe properties and ambient radiation properties. The stabilization of the radiometer amplification is based on the construction principle (Dicke) or on a regular reference load measurement (total output radiometry). Calibration of the measuring results can be implemented by known objects giving hot/cold-brightness temperatures or by artificial loads, for example, by placing before the radiometer antenna input a piece of space cloth having the ambient temperature and alternately a piece of space cloth cooled with e.g. liquid nitrogen. The side lobe properties of the antenna can be estimated by measuring the antenna beam figure at used frequencies. The ambient radiation properties can be estimated by known radiation properties of the ground. Based on this and the above mentioned references, the determination of the atmospheric brightness temperature absolute values requires complicated equipment, `scientific instruments` as well as difficult and expensive measurement systems.
The atmospheric weather phenomenons can be also observed by a radar. The effect transmitted by the radar scatters from the water drops thus revealing possible water containing objects. Use of the radar requires a transmitter/receiver equipment. The curvature of the ground causes a shadow region, which restricts the operation range of the radar. The currently used rain indicating detectors are based on the observation of some electric property change in the detector component caused by the rain (rain drops or snow flakes), for example, the capacitance or breakdown voltage. The mechanical constructions of these detectors are due to their operation principle open and therefore sensitive to malfunctions caused by fouling and require regular service.
One rain detector application area is the automatic control of, for example the heating systems of satellite earth station antenna reflectors. The wet snow gathered on the earth station antenna surfaces attenuates the signal and turns the antenna beam away from the satellite direction thus reducing the capacity of the antenna. The rain detectors function in connection with outdoor temperature detectors, controlling the reflector heating into function when it rains at the temperature area of approx. -5.degree.-+5.degree. C. At higher temperatures the reflector snow melts by itself and at lower temperatures there is the risk that the melted snow freezes to the antenna constructions. Dry frozen snow has also a smaller effect than wet snow.
In addition to the above mentioned mechanical construction disadvantages, the present rain detectors in the satellite earth station heating control system have the deficiency that they in certain conditions do not observe the problems caused by snow gathered on the antenna surface. Frozen snowfall (for example, 10.degree. C.) and its gathering on the reflector surface does not switch on the heating control system. The warming of the gathered frozen snow when the weather becomes warmer or the sun has warmed the reflector surface increases the liquid water content of the snow. The melting of the gathered frozen snow on the antenna surface might cause long breaks or quality deteriorations in the telecommunication for several days after the snowing.